Plaque fluid and saliva were collected from 25 children aged 7-12 years from two communities with contrasting levels of caries and assayed for secretory immunoglobulin A (SIgA), IgG, IgM, complement (C'3) and lysozyme. Significantly more IgA was found in the saliva and lysozyme in the plaque fluid from subjects with low caries experience. Subjects exhibiting antibody reactive with S. mutans serotypes a and d had a lower prevalence of caries than those who failed to demonstrate antibodies.